1. Field of the Invention
The present invention relates to a specific diisocyanate adduct which is blocked with .epsilon.-caprolactam and is based, in particular, on isophorone diisocyanate (IPDI), to a process for the preparation thereof and to the use thereof for producing light-resistant polyurethane resin powders (hereinafter PUR powders).
2. Discussion of the Background
Thermosetting powder compositions which are obtained by reacting a resin which contains hydroxyl groups with a masked polyisocyanate form part of the state of the art and are widely used. The isocyanates preferred for thermosetting powder compositions are (cyclo)aliphatic polyisocyanates, specifically because of their excellent ageing characteristics compared with the aromatic isocyanates which have, in particular, the disadvantage of yellowing on storing and ageing.
The PUR powder hardeners which have become particularly widely used are isocyanate-polyol adducts which are blocked with .epsilon.-caprolactam and are based on isophorone diisocyanate. The reason for this is very probably the great difference in the reactivity of the two NCO groups in IPDI, which allows for specific formation of adducts of IPDI with the polyol, providing a narrow molecular weight distribution. A narrow molecular weight distribution of the hardener is the precondition for good levelling of the cured powder.
IPDI blocked with .epsilon.-caprolactam melts at 53.degree.-55.degree. C. As a consequence of the low melting point, caking of the PUR powders prepared from this blocked IPDI occurs on storage. To increase the melting point, IPDI is subjected, before the .epsilon.-caprolactam blocking, to an increase in molecular weight, for example by chain extension with a polyol, or by partial trimerization and/or dimerization.
Specified as chain extenders for IPDI in DE-C 2,105,777 are polyols such as trimethylolpropane, 2,2,4(2,4,4)-trimethyl-1,6-hexanediol, diethylene glycol, in DE-A 2,542,191 are mixtures of di- and trifunctional polyols, and in U.S. Pat. No. 3,931,117 are trifunctional polyols. DE-A 2,735,497 describes the use of partially trimerized IPDI blocked with .epsilon.-caprolactam as PUR powder hardener; partially dimerized aliphatic diisocyanates, specifically IPDI, whose free-NCO groups are partially or completely blocked with .epsilon.-caprolactam are likewise described as PUR powder hardeners (DE-A 2,420,475). DE-A 3,143,060 describes PUR powder hardeners which are obtained by chain extension of IPDI with an aliphatic diamine and subsequent .epsilon.-caprolactam blocking.
It is common to all these PUR powder hardeners that they are straightforward to prepare in a one-pot process. On an industrial scale, there are limits to this one-pot process when the melting ranges of the blocked polyisocyanate adducts are in the vicinity of the unblocking temperature of the reaction product, because partial unblocking of the blocked polyisocyanate adducts must be expected when the reactor is emptied. It is furthermore known that, in the case of PUR powders, the chemical resistance (in the case of a polyol) improves as the functionality of the hardener increases. This association of the effect of the functionality of the blocked polyisocyanate adduct on the properties of the PUR powder coating was shown very impressively in the Powder Coatings Symposium in London on 30.10.1987 (R. Gras; H. Schnurbusch).
The PUR powder hardener with superior properties presented there has an NCO functionality of about 3.3 and a melting range of 145.degree.-148.degree. C. The preparation of a hardener of this type on an industrial scale is, as already described, impossible or possible only with very great difficulty in a simple one-pot process because of the high melting point. This is very probably the reason why this hardener, which makes it possible to produce powder coatings with excellent solvent resistance, is not yet commercially available.
The object of the present invention was thus to prepare blocked highly functional polyisocyanate adducts of good chemical resistance with a melting point &lt;120.degree. C., preferably 80.degree.-110.degree. C., by a simple process which can be carried out industrially.
It has surprisingly been found that compounds of this type with low melting point and good solvent resistance are obtained when the diisocyanate adduct employed for the blocking is reacted with the blocking agent, specifically .epsilon.-caprolactam, not in the NCO: (monofunctional) blocking agent equivalent ratio of 1:1 but in the ratio of 1:1.1-2. It was not predictable and particularly surprising that the excess of blocking agent which, after all, partly remains in the surface coating after hardening has no adverse effect on the coating surface. Exactly the opposite would have been expected.